1. Field of the Invention
The present invention relates generally to a distortion eliminating circuit, and is directed more particularly to a distortion eliminating circuit of the feed forward type for use with an amplifier.
2. Description of the Prior Art
In general, since an amplifier which forms an amplifying circuit used in an audio circuit is nonlinear, in order to eliminate distorted components from its output signal the amplifier is provided with a negative feedback loop. When a number of such negative feedback amplifiers are connected to increase the amount of the feedback, defects are created wherein the circuit is oscillated by the frequency characteristics, the time delay of negatively fed back signal and so on that great distortion is transiently generated.
In order to eliminate the distortion components without using the negative feedback technique, a distortion eliminating circuit has been suggested by the prior art which is of the feed forward type.
A prior art amplifier having a distortion eliminating circuit of the feed forward type will be explained with reference to FIG. 1. In the circuit of FIG. 1, an input signal S.sub.1 applied to an input terminal 1 is fed to an amplifier 10 formed of a power amplifier and thereby amplified. An output signal S.sub.2 therefrom is supplied through a resistor 4 to a load resistor 7. In FIG. 1, 30 designates a differential amplifier which is supplied with the input signal S.sub.1 and the output signal S.sub.2 which is same in phase as the input signal S.sub.1. Thus, the amplifier 30 differentially amplifies both signals S.sub.1 and S.sub.2, detects a difference therebetween and delivers a difference signal S.sub.4 between the signals S.sub.1 and S.sub.2. In this case, since the amplifying characteristic of the amplifier 10 is non-linear, the output signal S.sub.2 therefrom contains distorted components S.sub.D. Therefore, if the levels of the signals S.sub.1 and S.sub.2 are selected suitably, the difference signal S.sub.4 delivered from the differential amplifier 30 becomes the same as the distortion components S.sub.D from the amplifier 10.
The differential output S.sub.4 is fed through the resistor 6 to an adding point 5 and added to the output signal S.sub.2 supplied to the point 5, and then supplied to the load 7 as an output signal S.sub.3. In this case, since the distortion component derived from the differential amplifier 30 is opposite in phase to the distortion component S.sub.D contained in the output signal S.sub.2, when both signals S.sub.2 and S.sub.4 are added to each other at the point 5, the distortion components thereof are cancelled with each other. Therefore, the output signal S.sub.3 appeared at the point 5 and fed to the load 7 contains no distortion components. Thus, in the circuit of FIG. 1, the resistors 4 and 6 each serve as an adding resistor.
The reason why the signal S.sub.3 contains no distortion component or is an output with no distortion component will become more apparent from the following description. Now, it be assumed that the voltages of signals S.sub.1, S.sub.2, S.sub.3 and S.sub.4 are respectively taken as e.sub.1, e.sub.2, e.sub.3 and e.sub.4 ; the resistance values of resistors 4, 6 and 7 as R.sub.1, R.sub.2 and R.sub.L ; the currents flowing through resistors 4, 6 and 7 as i.sub.1, i.sub.2 and i.sub.3 ; the amplification degree of the amplifier 10 as A.sub.1 ; and that of the differential amplifier 30 as A.sub.2 as shown in FIG. 1. Then, the following equations (1) to (5) are respectively established. ##EQU1##
Accordingly, the output voltage e.sub.3 is expressed as follows: ##EQU2##
As described above, the amplification degree A.sub.1 is non-linear as well known. In this case, if it be assumed that the amplification characteristic of the differential amplifier 30 is linear since it processes the signal of a low level and that the first factor including A.sub.1 of the right side of the equation (5) is zero, the output voltage e.sub.3 contains no distortion component. Accordingly, in order to cancel the distortion components with each other, it is sufficient that the following equation (6) is established. ##EQU3##
In other words, if the amplification degree A.sub.2 is selected to satisfy the equation (6), the output with no distortion can be obtained at the point 5 irrespective of the amplification degree A.sub.1 of the amplifier 10.
The inventors of this invention already proposed amplifying circuits as shown in FIGS. 2 and 3 each of which can eliminate the distortion of the amplifying circuit having an output stage of the emitter follower type by a distortion eliminating circuit of the feed forward type such as shown in FIG. 1. In FIGS. 2 and 3, the references same as those FIG. 1 designate the same elements.
In the circuit of FIG. 2, an emitter follower type amplifier transistor 2 is used as the amplifier 10. The signal S.sub.2 obtained at the emitter of the transistor 2 is fed through the emitter resistor 4 and point 5 to the load resistor 7 as the output signal S.sub.3. In this case, the amplifying characteristic of the transistor 2 is non-linear, so that the output signal S.sub.2 from the transistor 2 is distorted.
In order to detect the distortion component, the differential amplifier 30 is used which is supplied at its non-inverted input terminal with the input signal S.sub.1 and at its inverted input terminal with the output signal S.sub.2. The output signal S.sub.4 from the differential amplifier 30 is added through the adding resistor 6 to the output signal S.sub.2 at the point 5.
With the circuit of FIG. 2, the differential amplifier 30 operates as the difference detecting circuit between the signals S.sub.1 and S.sub.2 and produces the signal S.sub.4, which corresponds to the opposite-phased distortion component contained in the signal S.sub.2 from the amplifier 10, so that the distortion components are cancelled with each other at the point 5. In this case, the amplification degree A.sub.2 of the differential amplifier 30 is of course selected also to satisfy the equation (6).
As set forth above, by selecting the amplification degree A.sub.2 of the differential amplifier 30 as (R.sub.2 /R.sub.1), the output signal S.sub.3 with no distortion can be obtained at the point 5 regardless of the amplification degree A.sub.1 of the amplifier 10. Accordingly, since the distortion can be positively eliminated without using negative feedback structure, there is no defect inherent to the negative feedback structure.
In the circuit of FIG. 3, the amplifier 10 shown in FIG. 1 is formed of a push-pull amplifier which consists of mainly NPN-type and PNP-type transistors 2 and 14 and 12 and 15 designate coupling capacitors. Emitter resistors 13 and 16 are respectively connected in series to the transistors 2 and 14. The connection point between the emitter resistors 13 and 16 is connected through the adding resistor 4 and point 5 to the load resistor 7. The signal S.sub.2 obtained at the connection point between the emitter resistors 13 and 16 and the input signal S.sub.1 are both supplied to the differential amplifier 30 and then differentially amplified, by which the distortion component is detected.
By the circuit of FIG. 3, the distortion component generated from the push-pull amplifier 10 is cancelled by the distortion component delivered from the differential amplifier 30 similar to the circuit of FIG. 2.
In the prior art amplifying circuits of FIGS. 2 and 3, the input signal level to the differential amplifier 30 becomes (1-A.sub.1)e.sub.1, so that the voltage e.sub.4 of the differential output signal S.sub.4 is expressed as follows: EQU e.sub.4 =(1-A.sub.1)A.sub.2 .multidot.e.sub.1 ( 7)
Where, since A.sub.1 is nearly 1 (A.sub.1 .div.1), the voltage e.sub.4 is very low in level. Therefore there may be such a fear that the differential amplifier 30 is driven by the output signal S.sub.3 having a high amplitude and appeared at the point 5 from the amplifier 10 and hence the output from the differential amplifier 30 is distorted. To avoid this defect, it is necessary to increase the power capacity of the differential amplifier 30. Further, in this case the power consumption of the drive current flowing through the resistor 4 can not be neglected, but this defect can not be avoided.
The current i.sub.2 flowing through the resistor 6 at this time can be expressed from the equations (4), (5) (6) and (7) as follows: ##EQU4##
In the above amplifying circuits of FIGS. 1, 2 and 3, since the outputs from the amplifier 10 and the differential amplifier 30 are composed or added, a pair of the adding resistors 4 and 6 are required. Although the adding resistor 4 is small in resistance value, it consumes rather large power since this resistor 4 is inserted into the path to supply the output current to the load.